The present invention relates to a receiver for coherent optical communication and, more specifically, to improvements in a phase diversity receiver for coherent optical communication.
The coherent optical communication system is suitable for long-distance signal transmission because its receiving sensitivity is higher than the current, practical intensity modulation direct detection system, and is suitable for bulk transmission owing to its capability of high-density multiplexing in transmitting signals using electromagnetic waves of frequencies in the frequency range of near visible light. The heterodyne system, the homodyne system and the phase diversity system are generally known as receiving systems for coherent optical communication. The phase diversity system, in particular, is suitable for high-speed transmission because the band width for the light wave detector (photoelectric converter) is half that for the photodetector of the heterodyne system. The phase diversity system, as compared with the homodyne system, is suitable for practical application because the phase diversity system does not require a light source with a very narrow spectral line width for emitting a carrier beam and a local-oscillator beam and does not need any phase locking circuit. However, whereas the heterodyne system is capable of compensating chromatic dispersion (group delay) in the optical fiber in the IF band (the intermediate frequency band), the homodyne system or the phase diversity system, which obtains signals directly in the baseband, is unable, in general, to compensate dispersion easily because the upper sideband and the lower sideband are folded. Accordingly, the phase diversity system needs improvements to enable the phase diversity system to compensate dispersion easily.
The heterodyne system mixes a received signal beam and a local-oscillator beam by the square-law function of the photodetector to produce an IF signal having a frequency, for example, a frequency in the microwave frequency range, corresponding to the difference between the frequency of the signal beam and that of the local-oscillator beam and demodulates the IF signal. In the heterodyne system, as well as in the foregoing other systems, the photodetector provides an IF signal having an amplitude proportional to the product of the amplitude of the received signal beam and that of the local-oscillator beam and, therefore, signals can be received at a high sensitivity by using a local-oscillator beam of an appropriate intensity. However, in the heterodyne system, a band for the photodetector is in the range of 0.5 B to 2.5 B when the intermediate frequency f.sub.IF is, for example, 1.5 times the bit rate B. Accordingly, when the bit rate is 10 Gb/s, the frequency band must be from 5 GHz to 25 GHz. Since it is difficult to provide a photodetector having a flat frequency response characteristic and a satisfactory noise characteristic in such a frequency band, the heterodyne system is not necessarily suitable for high-speed transmission. However, since the heterodyne system is able to achieve demodulation through envelope detection or the like, requirement of the light source relating to spectral line width is not very severe. Furthermore, in the heterodyne system, the upper sideband and lower sideband by modulation in an IF signal spectrum are not folded, the dispersion in an optical fiber can be compensated by using an equalizer employing a strip line.
In the homodyne system, the phase of the local-oscillator beam is controlled so that the carrier of the received signal beam and the local-oscillator beam are synchronized and a baseband signal is obtained directly without using an IF signal. Accordingly, similarly to the condition with the intensity-modulated direct detection system, the bit rate B satisfies a desired band for the photodetector. Accordingly, the homodyne system is suitable for high-speed transmission. However, the homodyne system needs an optical phase synchronizing loop and a light source with a very small spectral line width. Moreover, since the upper sideband and lower sideband of the signal are folded on the baseband, the homodyne system is unable to compensate dispersion by an equalizer, which is different from the heterodyne system.
In the phase diversity system, in general, a local-oscillator beam of a frequency slightly different from that of the carrier of the received signal beam is used. The received signal beam is mixed with the branched local-oscillator beam having a predetermined phase shift, for example, 90.degree., for modulation. Since the band for the photodetector of the phase diversity system may be substantially equal to that for the homodyne system, the phase diversity system is able to construct a high-speed system. The phase diversity system does not need the phase control of the local-oscillator beam, and hence does not need any light source capable of emitting light having a spectrum of very small spectral line width. However, the phase diversity system, similarly to the homodyne system, is unable to compensate chromatic dispersion in an optical fiber easily because the upper and lower sidebands are folded on the baseband.